In contrast to the asymmetric distribution
pattern for the canonical histone H3, the histone
variant H3.3 did not show this asymmetry during GSC divisions, by fluorescence images (Fig.
2, H to J) and by quantification (H3.3 GSC/GB
data in Fig. 2T, tables S1 and S2: H3.3-GFP
ratio in GSC/GB = 1.03; H3.3-mKO ratio in
GSC/GB = 1.03). The symmetry of the histone
variant H3.3 suggests that the asymmetric mode
is specific for canonical histone H3.

Fewer than 2% of all GSCs are undergoingmitosis; thus, all analyses above were based onpostmitotic GSC-GB pairs. To further examinethe histone segregation pattern during mitosis,we screened for mitotic GSCs. Indeed, old his-tones were mainly associated with the chromatidssegregated to the GSC side at metaphase (fig. S2),anaphase (Fig. 2, N to P, fig. S2, arrowheads),and telophase (Fig. 2, Q to S, arrowheads). Bycontrast, new histones were more enriched at thechromatids segregated to GB side (Fig. 2, N, P,Q, and S, and fig. S2, arrows). These results sug-gest that the sister chromatids preloaded with oldhistones are preferentially retained in GSCs andthat the ones enriched with new histones are par-titioned to GBs during GSC mitosis.